Polyoxymethylene (POM) is a homopolymer or copolymer prepared from formaldehyde, (a cyclic trimer of formaldehyde), or trioxane in combination with a comonomer such as a cyclic ether or a cyclic formal. POM has stabilized molecular terminals, and is blended with additives such as antioxidants or any other heat stabilizer in order to prevent is decomposition thereof.
A melamine-formaldehyde polycondensate is known as a useful additive, among others. The use of a combination of an antioxidant and a water-soluble melamine-formaldehyde polycondensate and the use of an antioxidant and a water-insoluble melamine-formaldehyde polycondensate are respectively proposed in German Patent Laid-Open No. 1694097 and Japanese Patent Laid-Open No. 33943/1977. These combinations improve the heat stability of the polyoxymethylene. However, further improvement is increasingly demanded in material properties, depending on the field in which the polyoxymethylene is utilized.
For example, reduction in malfunctions of mechanical parts for use in electrical or electronic equipment such as audio and video systems, has been demanded.
Illustratively, audio or video mechanical parts manufactured and assembled in a highly clean environment (such as a dust-free room) for the purpose of avoiding environmental contamination, are likely to suffer problems during practical operation when exposed to a high-temperature and high-humidity atmosphere, even though there is no environmental contamination attributed to dust, or the like This problem is due to additives contained in a rubber roller employed as a member of the audio or video mechanical part which bleed to the rubber surface and contaminate the magnetic head through the magnetic tape. The rubber surface disposed in the audio or video mechanical part thus exposed to a high-temperature and high-humidity atmosphere is cleaned as a temporary countermeasure against the above problem. This cleaning is effective to some extent. However, the countermeasure is not only never permanent but also consumes immense labor. Further, the use of a molded polyoxymethylene article together with a magnetic metal member, such as a magneto-optical disk or a metallized tape, in a relatively closed state may cause the latter to corrode. Therefore, a radical solution to the problem has been desired in the art.
The present applicants have found that the problems noted above are strongly connected with the quality of the materials comprising audio or video mechanical parts. That is, it has been found that conventional molded polyoxymethylene articles (which have mainly been utilized as mechanical parts such as chassis, gears and bearings tape cassette parts such as reels and guide rollers and as disk cartridge parts such as shutters and cartridge frames) cause bleeding of rubber parts (such as rollers and belts) and corrosion of magnetic metal members in an atmosphere in which the temperature and humidity are relatively high. The reason for such problems is that formic acid contained in the molded polyoxymethylene article is released into moisture of highly humid atmosphere and adheres to and penetrates into the rubber and magnetic metal members. In particular, it has become apparent that, in the case of the rubber, the formic acid reacts with the magnesium oxide present in the rubber, which is sparingly soluble in water, to thereby form magnesium formate readily soluble in water, which bleeds to the surface of the rubber and contaminates the magnetic head through the tape. Further, it has become apparent that, in the case of magnetic metal members, the formic acid itself corrodes the metal.
As mentioned above, polyoxymethylene has its heat stability improved by the addition of an antioxidant and various stabilizers. However, the occurrence of some thermal decomposition during molding cannot be avoided, so that formaldehyde and formic acid as polyoxymethylene decomposition products are contained inside the molded article. The formaldehyde is gasified and released into the environment when the molded article is heated to several tens of degrees centigrade or above, so that the amount thereof can be extremely reduced by heating the molded article at high temperatures or degassing it in a vacuum. However, the amount of formic acid cannot be significantly reduced even if it is heated at temperatures as high as one hundred and several tens of degrees centigrade or treated in hot water.
The present application have found that, not only can the amount of generated formic acid be extremely reduced, but also the heat stability of polyoxymethylene can be improved by blending a specified metallic compound with a polyacetal resin containing an antioxidant and a melamine-formaldehyde polycondensate. The present invention has been completed on the basis of the above finding.
According to the present invention, there is provided a polyoxymethylene composition prepared by blending 100 parts by weight of a polyoxymethylene with (a) 0.01 to 5.0 parts by weight of an antioxidant, (b) 0.01 to 5.0 parts by weight of a melamine-formaldehyde polycondensate and (c) 0.001 to 10 parts by weight of at least one metallic compound selected from among oxides and carbonates of magnesium and calcium. Further, there is provided a molded polyoxymethylene article produced by molding the above composition, characterized in that the amount of formic acid generated from the molded article is 1.0 .mu.g or below per square centimeter of the surface area thereof.